1. Field of the Invention
The subject invention generally pertains to refrigerant systems and more specifically to a system and method for avoiding carryover of liquid refrigerant from an evaporator to a compressor.
2. Description of Related Art
Refrigerant systems operating in a cooling mode typically include a compressor that forces refrigerant in series flow through a condenser for releasing heat from the refrigerant, a flow restriction (e.g., expansion valve) for cooling the refrigerant by expansion, and an evaporator where refrigerant therein vaporizes upon absorbing heat usually from a room being cooled or from some other cooling load. From the evaporator, the vaporized refrigerant returns to a suction side of the compressor to be recompressed and discharged back to the condenser to repeat the cycle.
If the refrigerant entering the compressor is not completely vaporized but instead has some entrained liquid refrigerant (known as “carryover”), one or more problems can result depending on the design of the refrigerant system. For some systems, high oil concentration in the evaporator promotes foaming and liquid carryover; the carryover introduces liquid refrigerant into the oil separator; liquid refrigerant in the oil separator reduces the separator's effectiveness; reduced separator effectiveness increases the oil concentration of the refrigerant; which in turn further increases the amount of oil in the evaporator; and that ultimately reduces the refrigerant system's efficiency and possible reduces the compressor's supply of oil.
For refrigerant systems that include a positive displacement compressor, such as a screw compressor, scroll compressor or a reciprocating compressor, carryover can damage the compressor, as liquid refrigerant is generally incompressible.
Although numerous liquid/gas separators have been developed to address the problem of carryover, such separators can add cost to the refrigerant system and can create an undesirable flow restriction between the evaporator and the compressor. Thus, there is a need for a better method of avoiding liquid carryover in a refrigerant system.
The present invention provides a method of controlling a refrigerant system to meet a cooling load that can vary from a range of lower loads to a higher load. The system includes a compressor that forces refrigerant in series through an expansion valve, an evaporator, and the compressor. The method includes the steps of: monitoring a thermodynamic variable associated with the refrigerant system; establishing a limit for the thermodynamic variable; comparing the thermodynamic variable to the limit to create a comparison; and, based on the comparison, selectively operating the refrigerant system in a normal operating mode and a capped operating mode. The method also includes the steps of: when operating the refrigerant system in the normal operating mode, controlling at least one of the compressor and the expansion valve so that the refrigerant system can address the cooling load within the range of lower loads; when operating the refrigerant system in the capped operating mode, controlling at least one of the compressor and the expansion valve in response to the thermodynamic variable so that the refrigerant system can at least partially address the cooling load at the higher load; and allowing the thermodynamic variable to vary more during the normal operating mode than during the capped operating mode.
The present invention also provides a method of controlling a refrigerant system to meet a cooling load that can vary from a range of lower loads to a higher load. The system includes a compressor that forces refrigerant in series through an expansion valve, an evaporator, and the compressor. The method includes the steps of: monitoring a primary thermodynamic variable associated with the refrigerant system; monitoring a secondary thermodynamic variable associated with the refrigerant system; establishing a limit for the secondary thermodynamic variable; comparing the secondary thermodynamic variable to the limit to create a comparison; based on the comparison, and selectively operating the refrigerant system in a normal operating mode and a capped operating mode. When operating the refrigerant system in the normal operating mode, the method includes the step of controlling at least one of the compressor and the expansion valve in response to the primary thermodynamic variable so that the refrigerant system can address the cooling load within the range of lower loads. When operating the refrigerant system in the capped operating mode, the method includes the step of controlling at least one of the compressor and the expansion valve in response to the secondary thermodynamic value so that the refrigerant system can at least partially address the cooling load at the higher load. The refrigerant system continues operating but does so at a restricted capacity that can help prevent the refrigerant from being carried over in a liquid state from the evaporator into the compressor when the refrigerant system is subject to the higher load.
The present invention further provides a system for controlling a refrigerant system. The system includes a cooling load that can vary from a range of lower loads to a higher load; a refrigeration system including a compressor that forces refrigerant in series through an expansion valve, an evaporator, and the compressor; apparatus for monitoring a thermodynamic variable associated with the refrigerant system; and apparatus for establishing a limit for the thermodynamic variable. The system also includes apparatus for comparing the thermodynamic variable to the limit to create a comparison; apparatus, based on the comparison, for selectively operating the refrigerant system in a normal operating mode and a capped operating mode; apparatus, when operating the refrigerant system in the normal operating mode, for controlling at least one of the compressor and the expansion valve so that the refrigerant system can address the cooling load within the range of lower loads; apparatus, when operating the refrigerant system in the capped operating mode, for controlling at least one of the compressor and the expansion valve in response to the thermodynamic variable so that the refrigerant system can at least partially address the cooling load at the higher load; and apparatus for allowing the thermodynamic variable to vary more during the normal operating mode than during the capped operating mode.